The present invention relates to a throwaway insert used for machining crankshafts and a pin mirror cutter using the same.
As a throwaway insert for a pin mirror cutter, one type is known in which eight corners are used as shown in FIGS. 6A-6C. With this throwaway insert (hereinafter abbreviated to insert), opposed two side faces 23 and 24 of a square negative insert are slightly inclined so that the corner angles at diagonal positions will be e.g. 85xc2x0 (FIG. 6C) and the corners where the side faces 23 and 24 intersect the top and bottom surfaces 21 and 22 are rounded. Four such inserts are mounted on a cutter body in a staggered manner to form peripheral cutting edges 25 and shoulder cutting edges 26 as shown in FIG. 7.
Japanese patent publication 11-197935 proposes use of two kinds of inserts which each use four corners. But in view of the tool cost, the type using eight corners is more preferable.
The insert of FIGS. 6A-6C has its shape limited to use eight corners and has to be arranged such that the axial and radial rake angles for both the peripheral cutting edges and shoulder cutting edges will be both negative. Thus it was difficult to achieve high efficiency and high accuracy of machining.
Also, its shape makes it difficult to distinguish between arcuate corner I and obtuse corner II (FIG. 6C), so that these corners tend to be mistaken. Besides, since the corner angles of a seating groove 27 for fitting an insert are all 90xc2x0 as shown in FIG. 8, there was a possibility of causing a mistake in corner mounting.
An object of this invention is to improve the cutting performance of the peripheral cutting edges, of which the work load is the highest, while allowing use of eight corners to reduce the tool cost.
Another object is to reliably prevent a mistake in corner mounting.
According to this invention, there is provided a throwaway insert for a pin mirror cutter, the insert being a negative insert having a substantially parallelogrammic shape, top and bottom faces each having an opposed pair of acute corners and an opposed pair of
obtuse corners;
an opposed pair of first side faces; and
an opposed pair of second side faces;
each of the first side faces being formed, as viewed from one of the second side faces, by one of a curved surface, a combination of flat surfaces connected to each other at angles and a combination of flat and curved surfaces; each of the first side faces being formed, as viewed from one of the top and bottom surfaces, by a substantially straight ridgeline cut away at acute corner side thereof so as to increase the corner angle of the acute corner; wherein arcuate surfaces having different radii of curvature from each other are formed at intersections between acute corner sides of the first side faces and the top and bottom faces and at intersections between obtuse corner sides of the first side faces and the top and bottom faces.
Preferably, the first side faces are formed, as viewed from one of the top and bottom faces, by two flat surfaces connected together at an angle, the substantially straight ridgeline being cut away so that the acute corner will have a first included angle of 70-85 degrees and a second included angle by 3-10 degrees smaller than the first included angle.
There is also provided a pin mirror cutter comprising a cutter body having a substantially cylindrical insert-mounting surface; and the throwaway inserts as claimed in claim 1 or 2, each of the inserts having an axis perpendicular to the top and bottom faces;
a first set of the inserts being mounted on the insert-mounting surface of the cutter body so as to be arranged substantially circumferentially of the insert-mounting surface in a staggered manner axially of the insert-mounting surface; the first set of the inserts being oriented such that the axis of the each insert will extend in the radial direction of the insert-mounting surface with one of the second side faces of the each insert serving as a rake face;
and further oriented such that curved and straight ridgelines at the acute corners of the respective inserts will form a peripheral cutting edge for machining a pin portion of a crankshaft, the peripheral cutting edge having a positive axial rake angle and a negative radial rake angle,
a second set of the inserts being mounted on the insert-mounting surface of the cutter body near both axial edges thereof;
and oriented such that the axis of the each insert will extend in the axial direction of the insert-mounting surface with one of the second side faces of the each insert serving as a rake face;
and further oriented such that the ridgelines at the obtuse corners of the respective inserts will form shoulder cutting edges for machining a counterweight portion of the crankshaft, the shoulder cutting edges having axial and radial rake angles that are both negative.
In this invention, because a negative insert having a parallelogrammic basic shape is employed, a required relief can be provided on the outer end side of the peripheral cutting edges by making positive the axial rake angle of the peripheral edges. Although the radial rake angle of the peripheral cutting edges become negative if the straight portions of the cutting edges are provided with a relief, by making positive the axial rake angle of the peripheral cutting edges, which are large in load, cutting performance improves, so that the machining efficiency and accuracy can be increased.
Also, because the corner angle of the acute corners is increased by cutting away part of the side faces to increase the corner angle of the acute angle corner, it is possible to ensure strength at the outer ends of the peripheral cutting edges.
Further, because the corner angles of the acute corner and obtuse angle corners are widely different, these corners can be easily distinguished from each other. Besides, because the insert seats for peripheral cutting edges and those for shoulder cutting edges have different corner angles, if the corners to be used are mistaken, mounting of the inserts itself is not permitted. This eliminates mistakes in the corner mounting.
Other features and objects of the present invention will become apparent from the following description made with reference to the accompanying drawings, in which: